The present invention relates in general to improvements in television receivers and more particularly to a novel circuit arrangement for identifying the horizontal scanning line of a transmitted signal containing a vertical interval reference signal.
Recently, the 19th horizontal scanning line of each field of video information of a transmitted television signal has been used to transmit a specified vertical interval reference (VIR) signal. The specified VIR signal contains components; namely, a chroma reference component, a luminance reference component and a black level reference component, enabling suitable signal processing circuitry within the television receiver to adjust or control the receiver's signal reproducing circuitry for optimizing the characteristics of the displayed image in response thereto. In particular, the VIR signal provides a facility for the automatic adjustment of the displayed image to compensate for imperfections in a received television signal in accordance with the chroma, luminance and black level reference components. Thus, it will be appreciated that a useful VIR control system must perform two essential tasks. Initially the system must include a capability for recognizing or identifying line 19 of each field of transmitted video information. Secondly, the components of the VIR signal on line 19 must be appropriately gated to suitable signal processing circuitry for controlling the receiver's image reproducing circuitry. It is to the former of these functions, i.e. line 19 recognition, that the present invention is largely directed.
Typical prior art line 19 recognition systems comprise a counter having an input connected for counting the horizontal flyback pulses developed in the horizontal deflection system of the receiver. Since each horizontal scanning line corresponds to one horizontal flyback pulse, the count developed by the counter can be conveniently decoded for identifying line 19. A major problem encountered by prior art systems of the foregoing type is that of repeatably initiating the counting sequence at corresponding times during each field of the transmitted television signal to insure a reliable line 19 recognition capability. In this regard, it will be appreciated that initiating the counting sequence with non-corresponding horizontal flyback pulses for successive fields of the television signal will result in inconsistent line 19 identifications. For example, assuming that a system is designed for initiating the counting sequence with the 5th horizontal flyback pulse of each field, erroneously initiating the counting sequence with the fourth or sixth flyback pulse will cause line 18 or 20 to be mistakingly decoded as line 19. Since only line 19 contains the transmitted reference information, the control circuitry will, depending on the design of the system, set the receiver in response to incorrect input data or be rendered inoperative altogether.
In order to overcome the foregoing problem, prior art line recognition systems, of which U.S. Pat. No. 3,780,218 to Rennick is exemplary, conventionally employ a circuit arranged for integrating the composite sync signal in order to appropriately initiate the counting sequence. The integrator is normally designed for developing an output signal achieving a particular threshold level at a predetermined point during the vertical sync pulse interval. In response to achieving this threshold level, a sensing circuit, such as a monostable flip-flop, is operative for enabling the counter for counting subsequently occurring horizontal flyback pulses. Typically, the integrator threshold voltage level is achieved during the fourth horizontal scanning line so that the counter is enabled for counting beginning with the horizontal flyback pulse corresponding to the fifth scanning line. In such systems, an output count of 15 would be decoded for identifying line 19. The foregoing process would, of course, be repeated for each field of the transmitted television signal whereby line 19 would be repetitively identified by initiating the counting sequence at corresponding points in the vertical sync pulse interval and suitably decoding the counter output.
While, under ideal conditions, the integrator type system referred to above performs adequately, it is often susceptable to noise which can severely reduce its effectiveness. For example, high frequency noise superimposed on the relatively broad vertical sync pulses results in an integrated output exhibiting excessive jitter and characterized by randomly occurring peaks and valleys along its slope. As a consequence, the count initiating threshold level is developed in an unpredictable manner whereby the counting sequence is likely to be initiated incorrectly. As previously explained, an erroneously initiated counting sequence will produce a faulty identification of line 19. Low frequency noise may also adversely affect the operation of an integrator initiated line 19 recognition system. In this regard, integrated low frequency noise often closely resembles the integrated vertical sync pulses. Therefore, the introduction of low frequency noise into the composite sync signal between, for example, two adjacent horizontal sync pulses may be interpreted by the recognition system as a vertical sync pulse interval such that the counting sequence would again be erroneously initiated. And, as before, the result is an incorrect identification of line 19. The present invention provides a line 19 recognition circuit largely overcoming the foregoing problems.